How Water Works

In its purest form, it's odorless, nearly colorless and tasteless. It's in your body, the food you eat and the beverages you drink. You use it to clean yourself, your clothes, your dishes, your car and everything else around you. You can travel on it or jump in it to cool off on hot summer days. Many of the products that you use every day contain it or were manufactured using it. All forms of life need it, and if they don't get enough of it, they die. Political disputes have centered around it. In some places, it's treasured and incredibly difficult to get. In others, it's incredibly easy to get and then squandered. What substance is more necessary to our existence than any other? Water.

At its most basic, water is a molecule with one oxygen atom and two hydrogen atoms, bonded together by shared electrons. It is a V-shaped polar molecule, which means that it's charged positively near the hydrogen atoms and negatively near the oxygen atom. Water molecules are naturally attracted and stick to each other because of this polarity, forming a hydrogen bond. This hydrogen bond is the reason behind many of water's special properties, such as the fact that it's denser in its liquid state than in its solid state (ice floats on water). We'll look closer at these special properties later.

Water is the only substance that occurs naturally as a solid (ice), a liquid and a gas (water vapor). It covers about 70 percent of the Earth for a total of approximately 332.5 million cubic miles (1,386 million cubic kilometers) [source: U.S. Geological Survey]. If you're familiar with the lines "Water, water, everywhere, nor any drop to drink" from the poem "The Rime of the Ancient Mariner," you'll understand that most of this water -- 97 percent of it -- is undrinkable because it's saltwater (see illustration on next page). Only 3 percent of the world's water supply is freshwater, and 77 percent of that is frozen. Of the 23 percent that is not frozen, only a half a percent is available to supply every plant, animal and person on Earth with all the water they need to survive [source: National Geographic].

So water is pretty simple, right? Actually, there ar­e a lot of things about it that scientists still don't fully understand. And the problem of making sure that enough clean, drinkable water is available to everyone and everything that needs it is anything but simple. In this article, we'll look at some of these problems. We'll also explore exactly what plants, animals and people do with water and learn more about what makes water so special.­

The World's Water Supply

Where does our water come from?

There's often discussion in the news of the world's dwindling water supply, but this isn't entirely accurate. The amount of water isn't diminishing, but the demand for it is steadily increasing. Some scientists believe that the world's population, currently at 6 billion, will double by 2050 [source: Cossi]. In addition, the amount of water that is clean and drinkable is steadily decreasing because of pollution.

For many people in industrialized countries, getting water is as easy as turning on a faucet, and it's rather inexpensive. But freshwater isn't evenly distributed throughout the world. More than half of the world's water supply is contained in just nine countries: the United States, Canada, Colombia, Brazil, the Democratic Republic of Congo, Russia, India, China and Indonesia source: World Business Council for Sustainable Development]. Urban areas, obviously, have a greater need for water beyond the basics for drinking and sanitation. But overpopulation in undeveloped countries means that many people don't even get the basics.

Most of the world's freshwater -- about 2.4 million cubic miles (10 million cubic kilometers) of it -- is contained in underground aquifers. The rest comes from:

Water distribution has everything to do with political boundaries, economic development and wealth. In Mexico City, for example, 9 percent of the population uses 75 percent of the available water, and a crumbling infrastructure means that up to half of the water supply is lost through pipe leaks and evaporation [source: Cossi].

Some countries don't have enough clean water for their rapidly growing populations, and they can't afford the infrastructure necessary to clean and transport it. For example, most people in China's cities suffer from water shortages, and most of China's groundwater, lakes and rivers are polluted. About 700 million Chinese people have access only to drinking water that does not meet standards set by the World Health Organization [source: WHO].

Countries in the Middle East use the least amount of water per person because there are so few natural sources of freshwater. In contrast, the usage of water is higher in the United States than in any other country, with around 60,000 cubic feet (1,700 cubic meters) of water used per person in 2002 [source: Organisation for Economic Co-operation and Development]. But even within the United States, there are some states and regions that don't contain enough water to supply their populations. Coastal regions of Florid have so much saltwater that they must have freshwater piped in from inland areas, which has led to political disputes over control of the water supply.

Water Regulation

This aerial photograph shows damage to the Aral Sea caused by diversion of its water.

In many areas, water is regulated and distributed by governments. In the United States, it's regulated by the Safe Drinking Water Act. However, government control isn't always in the best interests of all people. In the 1930s, to irrigate cotton fields, the Soviet government created canals to divert the rivers that fed the Aral Sea (located between Kazakhstan and Uzbekistan). As a result, the surface area of the sea has shrunk by more than 50 percent and its volume by 80 percent over the past 50 years [source: Swanson]. Its salinity increased and it became polluted with pesticides, fertilizer runoff and industrial waste. The loss of the sea meant the decline of the commercial fishing industry, which helped to send the region into poverty. The pollutants from the exposed seabed have been found in the blood of Antarctic penguins [source: Swanson].

Some regions have privatized their water distribution, which has often led to conflict. In the late 1980s, the United Kingdom sold its water boards (governmental water-supply organizations) to private companies, which improved the infrastructure. Many people were outraged that companies could profit off such a basic need, especially when people who could not pay their bills suffered harsh penalties. The problem was later remedied with legislation.

In 2000 and 2005, demonstrators took to the streets in Bolivia to protest the privatization of the water supply. When foreign companies took over Bolivia's water system, the cost of water became too expensive for the poor. In the city of El Alto, "the cost of getting a water and sewage hook-up exceeded a half-year's income at the minimum wage" [source: Shultz]. The 2000 revolt, called the "Bolivian Water Wars," led to martial law and 100 injuries. After both incidents, the Bolivian government canceled the private company contracts.

Currently, more than a billion people, about 17 percent of the world's population, don't have access to clean water [source: World Health Organization]. There are several governmental and nongovernmental organizations, including UNICEF and Water Aid, working to help poor communities in Asia and Africa obtain sustainable supplies of drinking water and sanitation facilities. Water shortages happen in the United States, too -- many states have programs to assist the disadvantaged with obtaining enough water and paying their water and sewer bills.

Obviously, lack of water is a big problem. But why is that, exactly? In the next section, we'll look at the part that water plays in the human body.

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Conserving Water

One way you can help reduce the demand for water is to conserve water year-round. Many areas ban watering your lawn and garden when water runs low, but here are a few easy things that you can do on your own:

Human Water Consumption

Why do humans need water?

Our bodies are about 60 percent water [source: Mayo Clinic]. Water regulates our body temperature, moves nutrients through our cells, keeps our mucous membranes moist and flushes waste from our bodies. Our lungs are 90 percent water, our brains are 70 percent water and our blood is more than 80 percent water. Simply put, we can't function without it. Most people sweat out about two cups of water per day (0.5 liters). Each day, we also lose a little more than a cup of water (237 ml) when we exhale it, and we eliminate about six cups (1.4 l) of it. We also lose electrolytes -- minerals like sodium and potassium that regulate the body's fluids. So how do we replace it?

We can get about 20 percent of the water we need through the food we eat. Some foods, like watermelon, are nearly 100 percent water. Although the amount of water that we need each day varies, it's usually about eight cups (2 l). But instead of worrying about getting in those eight cups, you should just drink when you start to feel thirsty. You can get your water by drinking other beverages -- but some beverages, like alcohol, can make you more dehydrated.

If your urine is dark yellow, you might not be drinking enough water. Of course, you need more water when you're exercising; ill with diarrhea, vomiting or fever; or in a hot environment for a long time. Most people can survive only a few days without water, although it depends on a number of factors, including their health and environment. Some have gone as long as two weeks. Followers of a Buddhist boy meditating in Nepal claim that he has gone two years without food or water, but doctors have not been able to substantiate this [source: All Headline News].

When you don't get enough water, or lose too much water, you become dehydrated. Signs of mild dehydration include dry mouth, excessive thirst, dizziness, lightheadedness and weakness. If people don't get fluids at this point, they can experience severe dehydration, which can cause convulsions, rapid breathing, a weak pulse, loose skin and sunken eyes. Ultimately, dehydration can lead to heart failure and death.

Dehydration caused by diarrhea is a major cause of death in undeveloped countries. Nearly 2 million people, mostly children, die from it each year [source: WHO]. Consuming water polluted with biological contaminants and not having access to adequate sanitary facilities can lead to diseases like malaria and cholera and parasites like cryptosporidiosis and schistosomiasis. Water can be also be contaminated with chemicals, pesticides and other naturally occurring substances.

On the next page we'll learn about purifying water.

Water Purification

Water that is safe to drink is called potable water, or drinking water, in contrast to safe water, which can be used for bathing or cleaning. In the United States, the Environmental Protection Agency sets maximum levels for the 90 most commonly occurring contaminants. If something happens to your water supply, your supplier has to contact you to let you know what precautions you should take.

Water treatment requires six basic steps.

In coagulation, coagulants like lime and alum are added to the water, which causes particulates to clump together.

Next, the water is shaken into larger clumps, called flocs.

The sedimentation process requires that the water stand for 24 hours, which allows the clumps to settle to the bottom.

The water is then filtered, disinfected (usually with chlorine) and aerated.

Aeration helps to remove certain contaminants like radon.

In the next section, we'll take a closer look at exactly how water circulates in animal and plant cells.

Water Survival Techniques

If your water becomes contaminated and you don't have bottled water, you can purify it in a few different ways. If it is cloudy, first filter it through clean cloths or allow it to settle and then pour off the clear water. Then, you can boil the water for one minute to kill most disease-causing organisms.

You can also add one-eighth of a teaspoon of household chlorine bleach per gallon of water (or follow directions on the label). You should double the amount if the water is discolored or murky. Stir and let it stand for 30 minutes. Chlorine bleach tablets are sold at camping supply stores to purify water for drinking. You can also use five drops of iodine per gallon to disinfect water.

Store boiled or disinfected water in clean, covered containers. If the boiled water tastes too flat or the chlorine taste is too strong, pour it from one container into another.

Plant and Animal Water Consumption

Water regulates the temperature of plants and transports nutrients through them.

Plants contain even more water than animals do -- most of them are anywhere from 90 to 95 percent water [source: BBC]. Just as it does in animals, water regulates the temperature of the plant and transports nutrients through it. But instead of taking in water by drinking and eating, plants get it through dew, irrigation and rainfall.

Plants take in water through their roots, and green ones use it in photosynthesis, which is how they create sugar for food. (You can learn more about the process of photosynthesis in How the Earth Works.) Plants also need water to support themselves. Pressure from the process of osmosis -- the movement of water from the outside to the inside of the plant's cells -- keeps up the plant's cell walls.

When you water a plant, it sucks up the water through capillary action. Then the water travels from the roots through tubes called xylem vessels. Water reaches the leaves of the plant and escapes through small holes called stomata, which open when the plant needs to cool down. This process is called transpiration and is similar to how people (and some animals) sweat. Carbon dioxide also enters the plant through the stomata.

Processing water is more complicated in animals and people, although it's also similar in a lot of ways. Water that you consume is absorbed in the upper small intestine through osmosis. It enters the bloodstream and is transported all over the body. Unlike plant cells, however, animal cells do not have cell walls. This is why animals have circulatory systems -- otherwise, our cells would absorb water and salt until they swelled. Our circulatory systems move water around our bodies and remove it as needed through sweating and urination.

A few animals, like a microscopic organism called the tartigrade, can go without water for an extraordinary period of time. If the tartigrade's environment doesn't have enough water, the animal goes into a life without water, called anhydrobiosis. Sugar takes the place of water in its cells, making it impervious to extremes in temperature. Its metabolism lowers, and the tartigrade stays at this barely alive state until it has enough water to really live again.

Some plants have also found unique ways to live with little or no water. One way is a variation of photosynthesis called Crassulacean Acid Metabolism (CAM) photosynthesis. In CAM photosynthesis, a plant stores carbon dioxide as acid and keeps its stomata closed during the day to save water (evaporation happens at a slower rate at night). It can even keep its stomata closed at all times if conditions are especially arid. Cacti use CAM photosynthesis to survive the extreme heat and drought of the desert.

Next, we'll look at how the hydrologic, or water, cycle functions.

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The Water Cycle

Water vapor that ends up in clouds eventually condenses into water droplets and precipitates as rain, sleet, hail or snow.

The water cycle is the continuous movement of water in and around the Earth. As previously mentioned, water never really goes away -- it just changes form. The sun drives the entire water cycle and is responsible for its two major components: condensation and evaporation. When the sun heats the surface of water, it evaporates and ends up in the atmosphere as water vapor. It cools and rises, becoming clouds, which eventually condense into water droplets. Depending on the temperature of the atmosphere and other conditions, the water precipitates as rain, sleet, hail or snow.

Some of this precipitation is captured by tree canopies and evaporates again into the atmosphere. The precipitation that hits the ground becomes runoff, which can accumulate and freeze into snow caps or glaciers. It can also infiltrate the ground and accumulate, eventually storing in aquifers. An aquifer is a large deposit of groundwater that can be extracted and used. This runoff also comes from snowmelt, which occurs when the sun and climate changes melt snow and ice. Finally, some of this runoff makes it way back into lakes and oceans, where it is again evaporated by the sun. You can learn more about the water cycle in How the Earth Works.

Water that falls to the ground and stays in the soil ends up evaporating and retiring to the atmosphere. But groundwater, which is the major source of our drinking water, can accumulate in aquifers over thousands of years. Unconfined aquifers have the water table, or the surface where water pressure equals atmospheric pressure, as their upper boundaries. Confined aquifers often lie below unconfined aquifers and have a layer of rock or other materials as their upper boundaries.

In the United States, the oldest groundwater, known as fossil water, is contained in the Ogallala Aquifer. Lying below about 175,000 square miles (450,000 square kilometers) of eight states in the Great Plains, the Ogallala Aquifer stores about 2,900 million acre-feet (3,600 million kilometers cubed) of water [source: High Plains/Ogallala Aquifer]. The Ogallala Aquifer was formed between 2 and 6 million years ago, when the Rocky Mountain chain was forming. Because the climate of the Great Plains is arid, water in the aquifer is being used faster than it can be recharged. That's why some scientists refer to using fossil water aquifers as watermining.

Groundwater may also exist on other planets. Images from the Mars Global Surveyor spacecraft show what looked like gullies carved out by rivers of water on the surface of the planet. According to NASA, the water is probably 300 to 1,300 feet (100 to 400 meters) below the surface. Europa, one of Jupiter's moons, may also have subsurface water. As our need for water outweighs the Earth's supply, scientists wonder if we may one day mine for water on the other planets and moons in our solar system.

Water has a lot of unique and amazing properties that make it so important to life. They're why we're constantly looking for better ways to obtain and conserve it. In the next section, we'll look at these properties and learn more about water itself.

Water Properties

The hydrogen bond between water molecules that we talked about in the first section is the reason behind two of water's unique properties: cohesion and adhesion. Cohesion refers to the fact that water sticks to itself very easily. Adhesion means that water also sticks very well to other things, which is why it spreads out in a thin film on certain surfaces, like glass. When water comes into contact with these surfaces, the adhesive forces are stronger than the cohesive forces. Instead of sticking together in a ball, it spreads out.

Water also has a high level of surface tension. This means that the molecules on the surface of the water are not surrounded by similar molecules on all sides, so they're being pulled only by cohesion from other molecules deep inside. These molecules cohere to each other strongly but adhere to the other medium weakly. One example of this is the way that water beads up on waxy surfaces, such as leaves or waxed cars. Surface tension makes these water drops round so they cover the smallest possible surface area.

Capillary action is also a result of surface tension. As we mentioned, this happens in plants when they "suck up" water. The water adheres to the inside of the plant's tubes, but the surface tension attempts to flatten it out. This makes the water rise and cohere to itself again, a process that continues until enough water builds up to make gravity begin pulling it back down.

Water's hydrogen bonds are also why its solid form, ice, can float on its liquid form. Ice is less dense than water because water molecules form crystalline structures at freezing (32 degrees Fahrenheit or 0 degrees Celsius) temperatures. The thermal properties of water are also linked to its hydrogen bonds. Water has a very high specific heat capacity, which is the amount of heat per unit mass required to raise its temperature by one degree Celsius. The energy required to raise the temperature of water by one degree Celsius is 4.2 joules per gram. Water also has a high heat of vaporization, which means that it can take a lot of heat without its temperature rising much. This plays a huge part in the climate, because it means that oceans take a long time to warm up.

Water is often known as the universal solvent, which means that many substances dissolve in it. Substances that dissolve in water are hydrophilic. This means that they are as strong or stronger than water's cohesive forces. Salt and sugar are both polar, like water, so they dissolve very well in it. Substances that do not dissolve in water are hydrophobic. This is the source of the saying "oil and water don't mix." Water's solvency is why the water that we use is rarely pure; it usually has several minerals dissolved in it.

The presence of these minerals is the difference between hard water and soft water. Hard water usually contains a lot of calcium and magnesium, but may also contain metals. Soap will not lather well in hard water, but hard water isn't usually dangerous. It can also cause lime scale deposits in pipes, water heaters and toilets.

Some of the latest controversy about water's properties lies in how ice behaves when it melts. Some scientists claim that it looks about the same as it does when it's solid, except that some of its hydrogen bonds are broken. Others claim that it forms an entirely new structure. So for all of its importance, we still don't completely understand water.

For lots more information about water and related topics, check out the links on the next page.

Hoversten, Paul. "Groundwater May Be Source for Erosion in Martian Gullies." NASA, December 6, 2006. http://www.nasa.gov/mission_pages/mars/images/pia09031.html

Jespergen, Kathy. "Safe Water Should Always Be on Tap." On Tap Magazine, National Drinking Water Clearinghouse, University of West Virginia, Spring 1997. http://www.nesc.wvu.edu/ndwc/HistSafeWater.html